Fibrous structures formed from a variety of materials, including natural and synthetic fibers in both staple and continuous form, woven or nonwoven, have long been known and used in filter operations. They are formed into a variety of shapes, e.g., cylindrical cartridge filters, and operate as depth filters.
A particularly useful filter of this type is disclosed in the pending U.S. application of Pall et al, Ser. No. 568,824, filed Jan. 6, 1984 (U.S. Pat. No. 4,594,202), and the corresponding published EPO Application Ser. No. 84309094.5 (Publication Number 0 148 638), the disclosures of which applications are incorporated herein by reference. The cylindrical filter elements disclosed therein comprise a fibrous mass of nonwoven, synthetic, polymeric microfibers. The microfibers are substantially free of fiber-to-fiber bonding and are secured to each other by mechanical entanglement or intertwining. The fibrous mass, as measured in the radial direction, has a substantially constant voids volume over at least a substantial portion of the fibrous mass. Preferably, it also has a graded fiber diameter structure over at least a portion thereof.
Depth filters function by mechanical straining of particles as they pass through the pores in the structure. In mechanical straining, particles are removed by physical entrapment as they attempt to pass through pores smaller than themselves. The filtering capability of such filter elements, therefore, is determined in large part by the lower limit on pore size.
Somewhat smaller pores can be formed by decreasing fiber diameter, e.g., at a constant voids volume finer fibers will yield smaller pores. Unfortunately, reducing the pore size, while it improves the filtering capability, increases the pressure drop and adversely affects filter life.
A filter may also remove suspended particulate material by adsorption onto the filter surfaces. Removal of particulate material by this mechanism is controlled by the surface characteristics of the suspended particulate material in the filter medium. Most suspended solids which commonly are subjected to removal by filtration are negatively charged in aqueous systems near neutral pH. This has long been recognized in water treatment processes where oppositely charged, cationic flocculating agents are employed to improve settling efficiencies during water clarification.
Colloidal stability theory can be used to predict the interaction of electrostatically charged particles and surfaces. If the charges of a particle in the filter sheet surface are of like sign and have zeta potentials of greater than about 20 millivolts (mV), mutual repulsive forces will be sufficiently strong to prevent capture by adsorption. If the zeta potentials are small, or more desirably, the suspended particles and the filter surface have opposite signs, the particles will tend to adhere to the filter surface with high capture efficiency. Thus, filter materials characterized by positive zeta potentials are capable of removing, by electrostatic capture, negatively charged particles much smaller than the pores of the filter.
Synthetic, polymeric microfibers of the type disclosed in U.S. application Ser. No. 568,824, e.g., polypropylene microfibers, have negative zeta potentials in alkaline media. Accordingly, their ability to remove negatively charged, suspended, particulate material by adsorption is limited. Additionally, they are hydrophobic. Thus, a filter comprising such microfibers, at a given applied pressure, has lower fluid flow rates than would an otherwise comparable filter comprised of hydrophilic microfibers. In other words, if hydrophobic microfibers are used, one must accept either a higher pressure drop across the fibrous mass or a reduced flow rate.
Synthetic, polymeric microfibers, though hydrophobic, do have desirable features. They are resistant to chemical attack. They also are clean, i.e., filter media migration is low. It would be highly desirable to retain the attractive features of polymeric microfibers, and of fibrous structures made therefrom, while obtaining the benefits of hydrophilicity and a positive zeta potential.
A process for treating normally hydrophobic, microfibrous, polymeric webs to form hydrophilic, microfibrous, polymeric filter sheets with positive zeta potentials is disclosed in Pall et al., U.S. patent application Ser. No. 397,762, filed July 13, 1982, and in the corresponding EPO Application Ser. No. 83.303952.2 (Publication No. 0 099 699), the disclosures of which are incorporated herein by reference. The process generally comprises:
(1) applying a first solution or dispersion of precipitating agent to a hydrophobic web comprised of polymeric microfibers to at least partially wet the web with the first solution;
(2) applying a second solution of a water-soluble, non-colloidal, cationic, thermosetting binder resin or polymer to the wetted web of step (1) above to form a web wetted with a mixture of the first solution or dispersion and the second solution;
(3) working the wetted web of step (2) above to mix the first solution or dispersion and the second solution, thereby facilitating the precipitation of the binder resin or polymer and the distribution in a uniform manner of the precipitated binder resin or polymer as a coating on the surfaces of the microfibers making up the worked web; and
(4) drying the coated web of step (3) above and curing the precipitated binder resin or polymer coating.
While that process provides excellent results with thin, flexible, fibrous, filter sheets, thin webs, and the like, for thicker three-dimensional structures characterized by structural rigidity, e.g., the fibrous cylindrical structures disclosed in U.S. patent application Ser. No. 568,824, it is less satisfactory since working of the structure to mix the first and second solutions in a uniform manner is difficult, if not impossible. The result can be uneven laydown of the coating since the working possible with thinner, non-rigid fibrous material which facilitates the precipitation of the binder resin or polymer and distribution of the precipitated binder resin in a uniform manner is not possible. As used herein, the term "structural rigidity" refers to the characteristic of three-dimensional microfibrous structures being insufficiently flexible to allow them to be worked or manipulated to an extent that a uniform distribution of binder resin can be accomplished as described above.
The present invention, then, is directed to fibrous structures, particularly cylindrical depth filters, and a process for their manufacture. Fibrous structures prepared in accordance with the method of this invention are hydrophilic and have positive zeta potentials. As a consequence of the hydrophilicity and positive zeta potential, they have reduced pressure drops at given flow rates as compared to hydrophobic counterparts and enhanced particulate stability for removal of negatively charged particulate material in fluid media.